After installing Thunderbird I found that although I had installed myspell-swedish from the package repositiory I didn't get Swedish as an option in Thunderbird. I checked the file list for the thunderbird package and it showed a directory called /usr/lib64/thunderbird/dictionaries. This directory contained symbolic links to /usr/share/myspell/en_US.aff and /usr/share/myspell/en_US.dic but not to the files for Swedish. Perhaps it would if I had installed the dictionary before thunderbird, don't know.
To fix this I opened a console in the /usr/lib64/thunderbird/dictionaries directory, and ran the following command:
sudo ln -s /usr/share/myspell/sv_SE.* .
After restarting Thunderbird I could choose the Swedish dictionary.